Hyaluronan (HA) is an abundant matrix component of the epidermis that influences the growth and differentiation of keratinocytes via specific interactions with cell-surface receptors and binding proteins. We have shown that epidermal levels of HA increase dramatically after cutaneous barrier injury, suggesting an important role in normal mechanisms of barrier repair. Abnormal response to barrier injury are involved in pathologic skin conditions, including atopic dermatitis, corn and callus formation, and hypertrophic thickening at the edge of chronic wounds. This grant will test the hypothesis that changes in HA amount, HA polymer size, and specific proteins associated with HA, are important for normal barrier recovery. For this purpose, we have developed a new in vitro model of barrier injury (acetone applied to 3-D cultures of rat keratinocytes) to study mechanisms of inducible HA and epidermal responses to barrier injury in vitro. Barrier injury in the model causes hyperplasia and stratum corneum thickening, mimicking the response in vivo. We show that the mechanism of increased HA requires the release of HB-EGF, activation of the epidermal growth factor receptor (EGFR) pathway, and activation of the Has enzymes which synthesize HA. Because CD44, the cell-surface receptor for HA, is well-known to interact and regulate EGFR/ErbB2 in cancer cells, the possibility of a self-amplifying feedback loop exists. We postulate that a positive autoregulatory feedback loop governs HA synthesis after barrer injury;however unlike in cancer, the feedback loop after injury is self-limited. Our hypothesis is novel and provides a strong framework for the investigation of the role of HA in epidermal hyperplasia. AIM 1 will establish the physical alterations in HA (fragment sizes, HA-adherent proteins) that occur after injury. AIM 2 will confirm an autoregulatory pathway in detail, with a series of experiments to investigate ligand release (HB-EGF), the activation of EGFR and its heterodimeric partners (ErbB2, ErbB3), the downstream activation of latent Has2 and Has3 enzymes via phosphorylation and/or via new enzyme synthesis, and the cross-talk between CD44 and EGFR. AIM 3 will directly address the question of whether HA must be present in the skin for barrier repair to occur normally. Experiments will employ mice in which alleles for the Has1, Has2, and Has3 genes have been deleted from the skin, in various combinations. The potential benefit to public health is a new understanding of the physiological role of HA in epidermal barrier repair. Findings from the research could provide new approaches for dampening epidermal hyperplasia and reducing abnormal cornification, thus providing new opportunities to ameliorate several very common skin diseases.